1. Technical Field
The present invention generally relates to the field of metrology, and more particularly, to scatterometry overlay measurements.
2. Discussion of Related Art
Current scatterometry targets and measurement procedures are designed to be symmetric under 180° rotations of the targets about a normal to the target. Typical symmetry operations do not flip the chirality of the coordinate system.
FIGS. 1A and 1B are schematic illustrations of prior art targets having a 180° rotational symmetry and the resulting inaccuracies in overlay measurements. FIG. 1A illustrates in a perspective view a target 90 having periodic structures with pitch P and exhibiting an overlay OVLx between the periodic structures at different layers 91, 92. The resulting measurement signal 95 is shown as spots at the imaging pupil plane (with axes NAx, NAy, NA standing for numerical aperture) which represent zeroth order diffraction image 80 and ±1 first order diffraction images 79, 81. Overlay measurements are derived from pupil points 85 which represent angular diffraction signals. In the illustration in FIG. 1A, where only overlay along the x axis exists, signals 79, 80, 81 are symmetric with respect to the pupil plane axis NAx. However, when the periodic structures of target 90 are segmented along the y direction as illustrated in FIG. 1B (e.g., to improve process compatibility, e.g., with smaller pitch p2<p1), a new overlay OVLy is introduced. Prior art targets 90 are designed to be symmetric to 180° rotations (a symmetry, which is denoted Rπ in the following) due to measurement considerations such as calibration of tool induced shifts which requires rotating the wafers. However, the Rπ symmetry mixes overlays along the x and y axes, as described in the derivation below (see Equation 3A), and the mixing results in an introduction of an error in the location and forms the diffraction images of the first order, illustrated schematically in FIG. 1B. The breaking of reflection symmetry in the Y direction, which gets mixed with the asymmetry in the X direction, thereby causing an error in the metrology reported overlay, is demonstrated in FIG. 1B to be a result of OVLy≠0. In general, however, such a symmetry breaking in the Y direction may be a result of many phenomena like asymmetric printing in the Y direction (e.g., an asymmetrically segmented bar or any asymmetrically designed and/or printed target features) or from light scattered along the Y direction from structures exterior to the metrology target.